1962 Magnavox Stereo Theater with a busted Theater...

[electra225]

My model 2MV418 Stereo Theater is an amazing device. A beautiful hardwood cabinet. The incomparable 93 series amp chassis, lots of bass, nice bright treble. Having remote control necessitated using 12" speakers, but they give little to nothing to the 15" ones in the non-remote jobs. This ST has the largest TV chassis available in 1962, a 27". It has a beautiful black and white picture. When it works. Therein lies the rub. It seldom works for more than maybe a couple times, then it goes on the fritz, usually while turned off. It has been totally recapped, new tubes populated. It likes to lose its high voltage. One time it was a dirty brightness control. I cleaned all the controls with De-Oxit, but apparently it doesn't work on Magnavox TV sets. One time, the cathode follower tube wasn't making good contact. Wiggling tubes restored operation. I'm overlooking something. TV repair isn't my gig, my diagnostic skills are tentative at best, then I'm terrified of high voltage. A better schematic is the first step. Then I am going to disconnect the plate lead on the Horizontal out tube and do a bit of voltage measurement. I want to clean the controls, yet again, very thoroughly and maybe multiple times. Ditto tube sockets where the tubes carry high current or pulse voltages. I need to make sure the horizontal oscillator is working. Ideas?

Thinking out loud here. What about the possibility of "tin whiskers" on one or more of the controls? The brightness, horiz and vert hold controls, and the contrast control are mounted on the underside of the top rear of the cabinet. This set had copious amounts of cigarette scum on it when I restored it. How does one check for tin whiskers, what tests do I need to do? Then, how do I fix the condition? This set has something odd going on.

[electra225]

While I'm waiting for the schematic to arrive, I tested the tubes. High voltage rectifier, horizontal output tube, the damper, the horizontal oscillator, and the cathode follower all check good. After I receive the schematic, I'll check B+ and boost voltages. Those being okay, I need to see if the horizontal oscillator is running.

[electra225]

I have had issues in the past with dirty controls. I removed the controls this afternoon to check them. The horizontal hold, vertical hold and brightness controls all check okay. They move smoothly and I didn't see any breaks in resistance. The contrast control, however, shows shorted to ground. It looks like there is a mica cap across this control. The schematic should clarify the situation and give me a value. I don't see how a shorted contrast control would give me a dark picture tube. It works in the video amp circuit and feeds the grid of the picture tube (I think)..... If this control is shorted, it might stop the video, but I wouldn't think it would cause a dark picture tube.

[electra225]

According to Mr. Bing, a shorted contrast control can cause a totally dark picture tube. If the control had gone open, that could also cause a totally black or a totally white screen, depending on how the circuit was wired. I need to remove the wires from that contrast control and investigate this more thoroughly. I said earlier that the contrast control feeds the grid of the picture tube. I think that should be the cathode of the picture tube is what it feeds. This set has that automatic fine tuning circuit, I forget the exact nomenclature at this moment. I wonder that that works with the contrast control circuit in some way.

[electra225]

At the risk of making a fool of myself on the Internet.....

Sometimes this is how we learn.....

I have a book of TV chassis schematics, the 1951 Beitman's edition which has been a surprisingly helpful text as I dig into TV chassis. I have seen two methods of how the contrast control works. One method is in the cathode circuit of the video amp. The other is in the grid circuit of the picture tube.

Let's assume that Mr. Bing's suggestion that a shorted contrast control can totally darken a picture tube, even though we have high voltage and scanning. Since I don't KNOW for sure, if or how this happens, let's continue on this line of thought for a moment. I can see how a shorted contrast control in the grid of the picture tube can darken the screen by cutting off the beam to the CRT. If the shorted contrast control is in the cathode circuit of the video amp tube, that would ground the cathode and shut off electrons from passing to the plate of the video amp. The video signal is impressed upon the grid of the video amp, passes to the plate, then on to the grid of the picture tube, typically pin 2. If you ground the cathode, that will kill the video, but will it also darken the screen? In theory, then, if the resistance of the contrast control decreases, it washes out the video. If it gets toward being grounded, it then kills the beam entirely, totally darkening the picture tube? I'm having a hard time getting my head around this theory......

[electra225]

We are fortunate to have a member named Paul on this forum who was good enough to scan and email a Magnavox factory document pertaining to the TV chassis in the Stereo Theater. This was several years ago, and we don't see much of him on the forum, but his contribution to the restoration of this instrument can't be overstated. This document bailed me out when I initially restored this instrument nearly ten years ago. It has potentially bailed me out yet again, today....

I am also the poster child for why one shouldn't attempt repair on any electronic device, and expect success, unless and until they utilize an appropriate schematic. I ran myself down a rabbit hole fumbling around without service information.

The value of the contrast control is 600 ohms. The other controls were in the "k ohms" range, so I was measuring the contrast control on too high of a range on my meter, thus showing a dead short. When I measured it CORRECTLY, it works and tests perfect. Now what? Why the dark picture tube? The resistor across the contrast control had a value of 1k ohms. With one end disconnected it checked good. I replaced it just for grins.

Reading the text in the document referenced above, I hadn't tested the 6EB8 video amp tube. This tube is a high-mu triode and a sharp-cutoff pentode. The pentode section is used as a video amp, the triode section used in the AGC circuit. The one in the chassis was a NOS/NIB RCA I put in when I restored the chassis. IF you remember, this set was populated with mostly Realistic Lifetime tubes. I got rid of all of them. I found the new RCA tube dead shorted. It is so severely shorted, I can't tell what elements are shorted for sure. I tested the Realistic Lifetime tube I took out and it checked wonderful. So in it went. Time will tell if this is the fix or not, but it is a definite problem. I checked and rechecked tester settings, so there is no mistake there. Next up, we connect everything and power up. Stand by.....

I saved every part I took off this ST when I restored it. They are in an ice cream container in a drawer. I save old parts on restorations so I can document something, or go back like it was if I need to. I started doing this in the auto restoration business. Old parts can come in handy on occasion. Maybe this time, too?

[electra225]

The shorted video output tube was the initial problem. I have the picture tube back, although I'm not out of the woods yet. There is no horizontal lock. Not even close. And I have a really dirty tuner. The reason I had no audio was due to the tuner. I had been babying the tuner since I got this thing going. Removing that tuner is going to not be fun. I have to remove the safety glass in front of the CRT, remove the mask, remove the remote control thingy, and God knows what else to get the tuner out. The big speaker on the tuner end of the cabinet has to come out, maybe the MPX adapter as well. I may have gotten one or both of the phase detector diodes in backwards. It's worse now than it was. I have to think about the tuner situation.

[electra225]

I think my next step is to dig into that tuner. That may be a part of the continuing issues I am having with this TV chassis. If it takes me a long time to get it done, it won't matter. I want this thing fixed. Any advice or direction would be much appreciated. Other than sending this thing to Shango or Bob Andersen, I don't see another option.

[electra225]

I got it going! What I did to fix it may be best discussed another time. I'm beat.....

[electra225]

I'm sorry I'm talking to myself here. I wanted to share this little victory with you guys. Days like this are why we mess with this old stuff. We get over our skis occasionally, taking on more than we have the ready knowledge to handle. We make mistakes, we chase our tail. We fumble and we guess, but, eventually, if we are diligent, the plan comes together and the old thing starts singing again. That is the second best feeling there is.

I decided that I needed to not get excited, treat this job like I'd never seen it before. Get some information, consult the schematic. Figure out if what you have found makes any sense on the schematic. The problem this whole time was two things. A tuner I didn't get clean the first time and a NOS tube that was failing. I won't repopulate a chassis with all new tubes again. I will use the ones that are there until it's proven they won't work. I won't get excited about brands. I have had two tubes of preferred brands fail and cause me untold grief. One a Telefunken, one an RCA. I will also fire up the Analyzer and complete all recommended tests and adjustments before I put the back on the cabinet. "Later" will never come. Also, I will never pull the back off the cabinet until I have the schematic laying on the bench. I should know better. I thought I was smarter than I was. I chased my tail and made a mess.

[TC Chris]

Earlier, you said "At the risk of making a fool of myself on the Internet..... ...." And I thought, "Oh c'mon, Greg, we're old guys and are way past false modesty or concerns about looking foolish." In a family email, I was writing about the worst days of the sciatica, when to mount the bicycle, I had to lay it flat on the ground, step carefully over, and lift it up because I could not swing a leg over. It's really undignified. But after wearing hospital gowns and getting annual prostate exams for years (bend over), the having multiple colonoscopies and two prostate biopsies, all involving nether regions, I've got no false dignity left, and only whatever real dignity I've earned. And remember that a certain member of this forum read the red-red-range resistor as red-red-red and delayed the fixing of his Heathkit for a couple weeks. And admitted it.

We all learn from others' mistakes as well as our own. Others' are less painless, so share.

Chris Campbell

[electra225]

I started this discussion on the "big forum", knowing there were more "TV guys" on there. They talk way above my head, get into stuff I'll never understand. They mean well, they had just as well been speaking Russian for all I understood what they said. I have to work on TV the simple way. I have to be logical and try not to overthink something. I dreaded getting back into this set because I thought the problem was going to be complicated. I didn't consider that the issue could be fairly basic. I had so much trouble with this TV I gave up hope of ever getting it to run again. I don't know that I'm out of the woods yet. It may not work again when I try to power up tomorrow. If it does, I'll try to run it once a week for at least an hour. That should keep it going and not let little things add up.

If you look at the first picture I posted, look in the lower right hand corner of the CRT mask. You will see the tuner is on channel 4. The DVD player I was using was on Channel 3. This shows how far the fine tuning was off on this chassis. The procedure is to set the fine tuning on each channel you will use. Then, with the automatic fine tuning feature of this chassis, the fine tuning will set automatically for each channel. When I changed the horizontal oscillator tube when I restored the chassis, that changed the horizontal frequency control setting. Now I have to reset that. There is no AGC adjustment on this chassis.

[William]

Congrats! Is that the Dick Van Dyke show on your screen?

Bill

[electra225]

It is. That is my test DVD. I fired the ST TV set up this morning. It came right up. I need to warm up the Analyzer and make a few adjustments, vertical height, horizontal width, maybe a touch on the focus. Watching an Andy Griffith DVD revealed that not all those old remastered DVD's are good quality. You don't want to condemn performance, nor do you want to praise performance, based on those things. I'll make determinations based on the Analyst then leave well enough alone. This stereo pulls almost 600 watts with the TV running. Seems like Magnavox could have switched the MPX off while the TV is running. We have 36 tubes hot with the TV running, won't get an Energy Star rating like that. The Magnalux feature of this set is amazing. It varies the brightness and contrast based on room lighting. When I walk past the set, the picture gets darker and lighter as I pass. The audio on this TV is amazing, never had a TV with that good of audio. The power transformer on the TV barely gets warm after running for five hours. The PT on the amp chassis, however, gets hot enough to fry eggs. I don't like that, but it doesn't seem to hurt it. Been like that ever since I've had the stereo. I'm really excited to have this thing working as well as it does. I need to consider making a modified back for the cabinet like I did for the Concert Grand. Not only does it have a "sparkly" TV chassis where the tube filaments glow brightly, but it puts out a lot of heat. No manufacturer was as good at producing sloppy wiring with mix and match components like Magnavox was. It's a wonder to behold. I can sit and look at the backside of this stereo for hours, amazed that it works at all.